A high performance instrumentation recorder typically has been used to record and playback high frequency standard pulse code modulated (PCM) serial digital data. In general, when analog data is converted to PCM data, it is often serialized into a single bit stream so that the data can be transmitted via a media such as a radio link and can also be recorded on a single channel of a tape recorder. The serialized streams of PCM data, when blocked into groups of words with a marker identifying the start of a group of data words, are called frames of data. The identifying marker is called a frame synchronizing code. A variety of modulation techniques are used to modulate the serialized data bits to improve reliability of the radio transmission and the reproduction of the signal when recorded. These techniques are referred to as PCM codes and are standardized. See, e.g., IRIG STANDARD 106-86 Telemetry Standards, ADA089908, published by the Range Commander's Council, U.S. Army White Sands Missile Range, New Mexico (May 1986).
PCM serial data, for example, is developed when testing systems such as guidance controls or sequence events devices which may have hundreds of parameters or events that must be monitored. A substantial test thus may acquire and record data samples taken from several hundred sensors in connection with the device being tested.
Tests of this magnitude may require more than one PCM encoder to convert analog data to digital data and the recorder would therefore utilize more than one record channel (one channel for each PCM data stream) to record all the data. This type of recorder and the associated tapes are usually very expensive. Moreover, these recorders are heavy, bulky, consume significant power and, when used to record high frequency data, are limited to as little as fifteen minutes recording time.
Not all tests require such a large amount of data which necessitates the use of an expensive conventional instrumentation recorder. Many tests require only a single PCM channel. In these circumstances, VCR recorders offer advantages over conventional instrumentation recorders.
VCR's, compared to conventional instrumentation recorders, are relatively inexpensive, small and capable of recording large quantities of data for several hours. However, VCR's have not widely been used in recording standard PCM serial data because of problems with data loss. Specifically, the standard television video format employed by VCR's includes vertical and horizontal synchronizing pulses which cause gaps in the data stream. These gaps create significant data bit dropouts which result in unacceptable losses of data in the recordation of high frequency continuous standard PCM serial data. For example, with the standard video format, a VCR used to record continuous PCM serial data would lose 4.7 microseconds of data every 63.5 microseconds and would further lose 190.5 microseconds of data sixty times a second. This is unacceptable when recording standard PCM serial data that involves a large number of parameters and/or events.
Another problem in using VCR's to record standard PCM serial data results from gaps or discontinuities in the data that may result as it is either being recorded or played back from the recording medium. VCR's use a helical-scan technique for recording where the tape is guided at an angle around the surface of a rotating drum in which the video record/playback heads are mounted. The record/playback heads cross the tape at a gradual angle from edge to edge. Space is left near each edge to record audio and timing signals on regular longitudinal tracks. The path taken by the video heads slants across the tape to form part of a helix.
During the record and playback procedures the video heads alternate from one to the other as they rotate on the drum. Head rotation is synchronized to the 60 Hz vertical synchronizing pulse in the video signal. A single scan of a head past the tape takes 1/60 second. The horizontal synchronizing pulses are used in automatic gain circuits of the VCR.
In an attempt to minimize the discontinuity caused as the recorder switches from one head to the next, an overlap in the data recorded on the tape is provided. That is, there is a period during which the same information recorded by the video head just leaving the tape is also recorded by the video head that is just beginning to make its pass at the tape. Electronic switching is used to change from one video head to the next so that a continuous signal is produced. However, if the heads are not perfectly aligned, the misalignment can cause the insertion of additional gaps or discontinuities in the PCM data as it is either recorded or played back from the tape.